1. Field of the Invention
The present invention relates to an apparatus for detecting the rotative velocity of a reel table, and more particularly to an apparatus for detecting the rotative velocity of a reel table of a tape recorder, in which a rotational state of a reel table is continuously detected for providing data to a control section of a driving mechanism to automatically stop the rotation of the reel table when the reel table is not normally rotated
2. Description of the Prior Art
Generally, tape recorders function for recording video and/or audio signals on a magnetic tape which travels along a tape travelling system, and reproducing the recorded signals. The magnetic tape is wound on a pair of wheels provided within a cassette. When the cassette is mounted onto a deck of the tape recorder in accordance with a loading mechanism, the pair of wheels are respectively mounted to a take-up reel and a supply reel installed on the deck. Upon the completion of the mounting, a predetermined travelling operation such as play, record, fast forward and rewind is carried out in accordance with a selected operation mode. At this time, the magnetic tape is released from one wheel to wind on the other wheel in accordance with the rotational direction of the take-up reel and the supply reel.
Meantime, the rotation of the take-up reel is probably obstructed by a defect of a deck mechanism or other factors during the travelling of the tape. Even under this state, the tape is continuously released in the direction of a head cylinder by a capstan, so that a slack of the tape occurs between the capstan and the head cylinder. If the cassette is ejected without realizing the occurrence of the tape slack, the tape is caught by surrounding elements within the tape recorder to be damaged, contaminated by grease coated on the surrounding elements or cut to be spoiled. In a video cassette tape, if a cassette unloading mechanism is carried out under the state of involving the tape slack, the tape is liable to be bitten between a cover and a chassis while the cover of the tape is closed.
Due to this fact, an apparatus for detecting the rotative velocity of the reel table is mounted on a lower end of the reel table to determine whether or not the reel table is normally rotated. Since the reel table is not normally rotated when the tape slack occurs, the apparatus for detecting the rotative velocity of the reel table detects the tape slack to provide data to a control section of a driving mechanism. The control section recognizing the slack forces to stop the rotation of the capstan and perform the eject or power-off operation.
FIG. 1 illustrates an apparatus of detecting the rotative velocity of the reel table according to one example of a conventional technique. A light emitting device 12 and a light receiving device 13 are arranged in parallel to each other on a lower portion of a reel table 11 spaced therefrom by a predetermined distance. A reflecting plate 14 formed with an alternately disposed reflection portion 15 and non-reflection portion 16 is installed to a lower plane of reel table 11.
If reel table 11 is rotated, light radiated from light emitting device 12 is reflected when reflection portion 15 of reflecting plate 14 places on an upper portion of light emitting device 12, so that light receiving device 13 receives the light. Whereas, when the non-reflection portion 16 places on the upper portion of light emitting device 13, light receiving device 13 cannot receive the light.
By this operation, a pulse is generated in view of an amount of the light monitored via light receiving device 13, and an interval between the pulses is constant in conformity with the rotative velocity of reel table 11. Therefore, the rotative velocity of the reel table is detected by measuring the interval of the pulses.
In the conventional apparatus for detecting the rotative velocity of the reel table, reflecting plate 14 is typically fabricated by a thin aluminum plate. Here, an upper portion of the thin aluminum plate corresponding to non-reflection portion 16 is printed with a black paint to be fabricated. Consequently, reflecting plate 14 is divided into reflection portion 15 consisting of the thin aluminum plate and non-reflection portion 16 of black color. Completely fabricated reflecting plate 14 is attached to the lower plane of reel table 11 by means of a both-sided tape.
The rotative velocity of reel table 11 is easily detected when utilizing reflecting plate 14 fabricated via the above-stated process, however, the number of elements is increased while complicating the fabricating process. In addition, it is burdensome to employ expensive light emitting device and light receiving device.
An apparatus for detecting the rotative velocity of the reel table according to another example of the conventional technique as shown in FIG. 2 does not utilize the reflecting plate considering the above-mentioned problems.
Here, a light emitting device 22 and a light receiving device 23 are opposed to be spaced from each other on a lower portion of a reel table 21. Also, a plurality of light shielding plates 27 protrude from reel table 21 on a lower plane thereof while being integrally formed with reel table 21, in which light shielding plates 27 are downwardly bent and regularly spaced along a circumference.
When light shielding plate 27 is placed between light emitting device 22 and light receiving device 23 while reel table 21 is rotated, light receiving device 23 cannot receive the light from light emitting device 22. Meantime, if a space between light shielding plates 27 is placed between light emitting device 22 and light receiving device 23, light receiving device 23 is permitted to accept the light from light emitting device 22.
By doing so, a pulse is produced in correspondence with the light sensed via light receiving device 23, and an interval of the pulse is varied in view of the rotative velocity of reel table 21. As the result, the rotative velocity of the reel table can be detected by measuring the pulses.
The above apparatus for detecting the rotative velocity of the reel table is simpler than the apparatus utilizing the reflecting plate, but employs the expensive light emitting device and light receiving device due to the optical detecting system like the apparatus as shown in FIG. 1. Therefore, it is burdensome on the economic basis. Furthermore, in actual practice, a distance between the light shielding plate and the sensors is narrow to heighten the rate of poor operation due to an inferior contact.